1. Field of the Invention
This invention relates to a method and apparatus for converting paraffins to higher hydrocarbons, such as gasoline range or distillate range fuels. In particular, it relates to methods and apparatus which combine the operation of catalytic dehydrogenation of a paraffinic feedstock to produce olefins with the operation of a multiple-stage catalytic reactor system to convert olefins to gasoline and higher hydrocarbons, and with downstream separation units to optimize heat recovery and product selectivity.
2. Discussion of the Prior Art
The conversion of paraffins, such as propane and butane, to mono-olefins, such as propylene and butylene, has been accomplished by thermal or catalytic dehydrogenation. A general discussion of thermal dehydrogenation (i.e., steam cracking) is presented in Encyclopedia of Chemical Technology, Ed. by Kirk and Othmer, Vol. 19, 1982, Third Ed., pp. 232-235. Various processes for catalytic dehydrogenation are available in the prior art. These processes include the Houdry Catofin process of Air Products and Chemicals, Inc., Allentown, Pa., the Oleflex process of UOP, Inc., Des Moines, Ill. and a process disclosed by U.S. Pat. No. 4,191,846. The Houdry Catofin process, described in a magazine article, "Dehydrogenation Links LPG to More Octanes", Gussow et al, Oil and Gas Journal, Dec. 8, 1980, involves a fixed bed, multi-reactor catalytic process for conversion of paraffins to olefins. Typically, the Houdry Catofin process runs at low pressures of 5-30 inches of mercury absolute, and high temperatures with hot reactor effluent at 550.degree.-650.degree. C. Dehydrogenation is an endothermic reaction, so it normally requires a furnace to provide heat to a feed stream prior to feeding the feed stream into the reactors. The UOP Oleflex process, disclosed in an article "C.sub.2 /C.sub.5 Dehydrogenation Updated", Verrow et al, Hydrocarbon Processing, April 1982, uses stacked catalytic reactors U.S. Pat. No. 4,191,846 teaches the use of group VIII meta containing catalysts to promote catalytic dehydrogenation of paraffins to olefins.
Recent developments in zeolite catalysts and hydrocarbon conversion methods and apparatus have created interest in utilizing olefinic feedstocks for producing heavier hydrocarbons, such as C.sub.5.sup.+ gasoline or distillate. These developments have contributed to the development of the Mobil olefins to gasoline/distillate (MOGD) method and apparatus.
In MOGD, olefins are catalytically converted to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as a ZSM-5 type catalyst. Process conditions can be varied to favor the formation of either gasoline or distillate range products. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in combination with paraffinic components, into higher hydrocarbons over a crystalline zeolite catalyst. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992. Marsh et al, in U.S. Pat. No. 4,456,781, have also disclosed improved processing techniques for the MOGD system.
The conversion of olefins in a MOGD system may occur in a gasoline mode or a distillate mode. In the gasoline mode, the olefins are catalytically oligomerized at high temperature ranging from 400.degree.-800.degree. F. and moderate pressure ranging from 10-1000 psia. To avoid excessive temperatures in the exothermic reactor, the olefinic feed may be diluted. In the gasoline mode, the diluent may comprise light hydrocarbons, such as C.sub.3 -C.sub.4, from the feedstock and/or recycled from debutanized product. In the distillate mode, olefins are catalytically oligomerized at high temperature ranging from 350.degree.-600.degree. F. and higher pressure ranging from 100-1000 psia than the gasoline mode. In the distillate mode operation, olefinic gasoline may be recycled and futher oligomerized, as disclosed in U.S. Pat. No. 4,211,640 (Garwood and Lee). U.S. Pat. No. 4,433,185 (Tabak) teaches the use of a two-stage catalytic oligomerization system in which a first stage operates in the distillate mode and a second stage operates in the gasoline mode.
Olefinic feedstocks may be obtained from various sources, including from fossil fuel processing streams, such as gas separation units, from the cracking of C.sub.2.sup.+ hydrocarbons, from coal by-products and from various synthetic fuel processing streams. U.S. Pat. No. 4,100,218 (Chen et al) teaches thermal cracking of ethane to ethlyene, with subsequent conversion of ethylene to LPG and gasoline over a ZSM-5 type zeolite catalyst.
Although heavier hydrocarbons, such as gasoline and distillate, can be produced from propane and butane by the prior art using dehydrogenation integrated with MOGD, there are several problems with integrating these processes, particularly to produce distillate. For example, U.S. Pat. No. 4,413,153 (Garwood et al) discloses a system which catalytically (or themally) dehydrogenates the paraffins to olefins, and then reacts the olefins by catalytic oligomerization (MOGD), to distillate range material. Catalytic oligomerization in the distillate mode is a high (preferably greater than 600 psig) pressure process, whereas dehydrogenation is favored by lower (less than 25 psig) pressure. Also, the dehydrogenation zone effluent is typically in vapor form. As a consequence, a compressor is required for pressurizing the effluent to feed a catalytic oligomerization reactor zone operating in the distillate mode, thus resulting in expensive compression costs. It would be preferable to feed a catalytic oligomerization reactor zone, particularly if operating in the distillate mode, as a liquid. U.S. Pat. No. 4,413,153 (Garwood et al) also provides a liquid feed to a catalytic oligomerization reactor zone by separating the dehydrogenation zone effluent in a separation zone to form a C.sub.2.sup.- gaseous stream and a C.sub.3.sup.+ liquid stream. Typically, such a separation is accomplished in a refrigerated distillation column. However, it is energy inefficient to feed gases at temperatures greater than 100.degree. F. to a refrigerated distillation column and then heat the C.sub.3.sup.+ liquid stream produced by distillation to greater than 350.degree. F. prior to feeding to catalytic oligomerization.
Moreover, dehydrogenation produces a dilute olefinic stream comprising 20-50% C.sub.3 /C.sub.4 olefins and the remainder comprising C.sub.3 /C.sub.4 paraffins. It is desirable to separate the olefins from the paraffins prior to feeding the olefins to a distillate mode catalytic oligomerization reactor zone. This separation reduces the rate and pressurization requirements of the distillate mode feedstream and facilitates recycle of paraffins to the dehydrogenation zone. At present, expensive gas plant separation is required to separate C.sub.3 /C.sub.4 olefins from C.sub.3 /C.sub.4 paraffins.